Study: Surface-transit would clog regional traffic

BY SCOTT GUTIERREZ, SEATTLEPI.COM STAFF

Published 9:19 pm, Monday, July 11, 2011

The state's plans for a tolled deep-bore tunnel to replace the Alaskan Way Viaduct would bring slightly more traffic congestion to downtown Seattle than a surface-transit concept favored by Mayor Mike McGinn, according to an analysis in the tunnel project's Final Environmental Impact Statement.

But a tunnel would get drivers through downtown by as much as 19 minutes faster than a surface-only Alaskan Way with more stoplights, according to the report. Even if a surface-transit replacement wouldn't result in more downtown gridlock than a deep-bore tunnel, as tunnel supporters have argued, it would lead to more regional traffic congestion as drivers look to avoid downtown Seattle, according to the state's analysis.

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"While the (surface-transit) concept may yield some benefits that are not associated with the Tolled Bored Tunnel, it would substantially reduce north‐south transportation capacity and would substantially increase travel times for north‐south trips, particularly through‐trips," according to the report.

A surface option would better serve local trips into downtown Seattle but would hurt traffic flow and freight mobility in the four-county Puget Sound region, according to the report. That is a main reason why the surface option is not considered viable by the Washington State Department of Transportation.

"The bored tunnel is viewed as an important corridor to support the regional economy and regional growth," said Ron Paananen, the viaduct replacement project administrator, during a Friday news conference.

The environmental impact statement, which analyzes a 1.7 mile tunnel beneath downtown as the state's preferred option for replacing the viaduct, was released last week with estimated traffic volumes on state Route 99 in 2030. The tunnel would carry about 57,000 vehicles per day by then, less than the 110,000 vehicles that use the viaduct.

That's because state lawmakers plan to toll the tunnel to raise $400 million for the overall $3.1 billion viaduct replacement project. Drivers who don't want to pay will use other downtown streets or Interstate 5, with most diversion occurring outside rush hour.

McGinn and environmental groups have pointed to diversion as another reason not to replace the viaduct with a tunnel and instead mitigate the traffic by augmenting I-5, city streets and increased transit service. With no place for all 110,000 vehicles to go, speeds would decrease and fewer drivers would travel through Seattle's city center, resulting in less traffic, according to an analysis in the environmental assessment.

But a tunnel would preserve capacity and still best serve trips through downtown Seattle, which make up about 50 percent of the viaduct's current traffic, Paananen said.

For example, a trip from Woodland Park to South Spokane Street in Sodo would take about 16 minutes with the tunnel, compared to an estimated 25 minutes under the surface-transit option, according to the report.

A morning rush-hour trip from West Seattle to downtown, however, would only be about three minutes faster - 32 compared to 35. The reason the tunnel isn't predicted to perform as well for West Seattle residents is due to expected congestion at the south portal caused by drivers exiting to avoid paying tolls. It would be 26 minutes if the tunnel weren't tolled.

"Diversion is a challenge and one we will have to manage as the project becomes operational," Paananen said.

But in terms of vehicle hours of delay (VHD) -- a measurement of traffic congestion -- the surface-transit option would result in 35,100 VHD compared to 38,700 VHD in Seattle's city center, according to the state's analysis. Regionally, however, a surface option would result in more 1.37 million compared to 1.36 million.

After 10 years of discussion, state leaders say they don't plan to revisit the tunnel decision.

The state and city plan to appoint a tolling advisory committee to plan how to address diversion and the effects on city streets. One option could be highway signs informing drivers of their estimated delay if they use another road instead of the tunnel.

There are many wildcards in the state's traffic modeling. One, the FEIS assumes a high-variable toll scenario of up to $4 each way at peak hours. That would be higher than necessary to finance the project, but WSDOT chose a conservative figure for the purposes of the Environmental Impact Statement.

Another issue is traffic models don't factor in possibilities such as tolls on Interstate 5, which transportation officials have discussed, or the effects of expanded light-rail service.

"I think we've acknowledged that we believe a lot will change between now and when the tunnel opens but we don't know exactly what that is or exactly what will change between now and 2030," Paananen said.

The tunnel would have no downtown exits. Instead, there will be a north portal near Mercer Street and Sixth Avenue and a south portal near the sports stadiums. The tunnel plan always was envisioned as working best with enhanced transit for people traveling into downtown, Paananen said.

"What (the tunnel) does not serve as well are trips to downtown Seattle," he said. "That's why the tunnel alternative is not a full-capacity replacement for the viaduct but it serves those trips that are not served as well by transit in the future."

WSDOT has not lost sight of the fact that King County would need a funding source to add more transit, he said.

The traffic modeling assumes growth in the regional economy and about 1.5 million more residents to the Puget Sound area, along with increased traffic. In 2015, the tunnel would carry about 45,000 vehicles daily, according to state estimates.

The tunnel would have capacity for more than 90,000 vehicles daily.

It would pick up the majority of growth and the "lion's share of demand for through trips" by 2030, Paananen said.

The Environmental Impact Statement will be reviewed for a final decision on the project. That's expected from the Federal Highway Administration next month.